It is known to coat a variety of substrates (e.g., metal, plastic, ceramic, etc.) with a variety of coatings (e.g., metal, paint, etc.) for engineering and aesthetic purposes. These coatings may be deposited by brush, spray, electrolysis, electrophoresis, dipping, etc. Over the years, a variety of well-known adhesion tests have been devised for evaluating the strength of the bond between the coating and the substrate. Common such tests include: (1) the "tape"test wherein adhesive tape is applied to the coating and then pulled off normal to the surface to see if the coating pulls off with the tape or adheres to the substrate; (2) the "bend"test wherein a test sample is bent to a predetermined angle (e.g., 90 degrees) to see if the coating cracks and bonds at the bend site; (3) the "stud pull"test wherein the coating is applied to a stud with an adhesive and the tensile stress required to separate the coating from the substrate measured; (4) the "shear"test wherein the coated surfaces are adhered together and shear stress applied thereto until failure occurs; (5) the "microscratch"test wherein the coating is scratched with a particular tool under a particular load, and the scratch produced characterized by a tool friction measurement, and acoustic emission detection and microscopy; and (6) the "glass bead"test wherein a stream of air-borne glass beads (e.g., SiO.sub.2, A12.sub.2 0.sub.3, or mixtures thereof) are impinged upon the coated part at a specified intensity which causes delamination and/or blistering of coatings which have unacceptable adhesion to their substrate. The "glass bead"test is used most frequently in connection with production lines to test a significant percentage of the parts coming down the line to screen out parts having unacceptable adhesion. The production line test involves: (1) cleaning of the parts to be tested so that the glass beads do not become contaminated; (2) positioning the parts a predetermined stand-off distance (e.g., 4 inches) from the nozzle from which the glass beads emanate; (3) setting the gas pressure (e.g., 70 psi) for the air used to propel the glass beads; (4) impinging the beads against the parts to be tested for a predetermined time (e.g., 4 seconds); (5) cleaning the glass beads from the tested parts; and (6) recycling the glass beads for reuse. Among the disadvantages of this process are the general undesirability of dealing with air-borne ceramic beads in a plant environment, time consuming and costly pre-cleaning and post-cleaning of the parts before and after testing, and wear on the equipment used to handle and propel the glass beads.
It is known in the art to provide internal combustion engine aluminum pistons with scuff and wear resistant metal coatings. Such coatings as iron, nickel-tungsten (3% w), nickel-cobalt (3% Co) or composite coatings comprising ceramic particles/fibrils (e.g., SiC) electrolytically codeposited with a metal, e.g., nickel, are known. Sprayed or vapor deposited coatings are also known. It is desirable to test a significant number (e.g., 10% or more) of coated pistons to insure that the deposition parameters consistently yield adequate coating adhesion so that no unsatisfactory pistons are assembled into an engine since the cost of engine repair and/or piston replacement after the engine is assembled and installed in a vehicle is so costly.
It is an object of the present invention to provide a simple, reliable, cost effective process for determining the sufficiency of adhesion between a coating and its substrate which process can be used as a quality control tool to "go/no-go" test and evaluate a significant percentage of parts travelling down a production line, and all without need for pre-cleaning and post-cleaning of the parts before and after testing and/or cleaning and recycling of ceramic media used in the test. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.